The present invention relates to photomultiplier tubes and, more particularly, to a photomultiplier tube in which a cage assembly is accurately and expeditiously positioned relative to a shield cup.
In the manufacturing of a high volume photomultiplier tube, conflicting objectives are frequently encountered. The tube must be designed to provide accurate and reproducible measurement of the phenomenon being observed, and yet, the tube must be inexpensive to produce. The former objectives demand that the position of each element of the tube be fixed accurately with respect to the other tube elements; however, this must be done without the use of a large number of expensive precision parts. In order to achieve the latter objective, the tube must be easy to assemble in order to eliminate the need for difficult, time consuming assembly techniques.
U.S. Pat. No. 4,370,585, issued to G. N. Butterwick on Jan. 25, 1983, shows a dynode assembly attached to a shield by means of a primary dynode and a plurality of interconnected tube elements which, when welded together, affix the shield to the dynode assembly. In the patented structure, the primary dynode is attached to the shield; however, in many tubes, it is desirable to operate the shield at a potential different from that applied to the primary dynode. In such a structure, such a mode of interconnection between the shield and the dynode assembly is nonavailing since the primary dynode must be isolated from the shield.
U.S. Pat. No. 4,426,596, issued to G. N. Butterwick on Jan. 17, 1984, shows a photomultiplier tube which utilizes a conventional structure for attaching a cage assembly to a shield cup. A pair of support rods extend through apertures in the dynode support spacers adjacent to the interface between the spacers and the shield cup. A pair of wire brackets are welded at the ends to the ends of the support rods extending through the apertures in the support spacers. The center portions of the wire brackets are shaped to conform to the bottom surface of the shield cup and are welded thereto to attach the cage assembly to the shield cup. The described structure requires a great deal of welding and tends to stress and distort the parts.
U.S. Pat. No. 4,431,943, issued to R. D. Faulkner et al. on Feb. 14, 1984, discloses a photomultiplier tube having an electron multiplier attached to a cup-shaped field forming electrode. In this structure, dynode support insulators extend through the bottom surface of the electrode and are locked in place by means of welded tabs and support rods extending through apertures in one side of the insulators, similar to those described in U.S. Pat. No. 4,426,596 to Butterwick, referenced above. In addition, locking slots are formed in the other side of the insulators to engage the bottom surface of the electrode. A great many parts are needed to affix the cage assembly to the electrode, and the assembly is complex and costly.